Pellet of iron ore and flux, and method for making same



E. W. PRICE June 8, 1965 PELLET OF IRON ORE AND FLUX, AND METHOD FORMAKING SAME 2 Sheets-Shet 1 Original Filed Oct. 5. 1961 COATING E R O CE. W. PRICE June 8, 1965 PELLET OF IRON ORE AND FLUX, AND METHOD FORMAKING SAME 2 Sheets-Sheet 2 Original Filed Oct. 5, 1961 United StatesPatent 3,188,195 PELLET {3F mfihl DRE AND FLUX, hflil'll lfiD FERlviAKlNG SAME Eugene W. Price West Allis, Wis, assigncr to-Allis-Chalmers Manuracturing Company, Milwaukee, Wis. Uriginal applicationGet. 5, 1961, Ser. No. 143,695. Divided and this application Nov. 26,1%3, Ser. No.

4 Claims. (Cl. 75-5) This invention relates to a method for making ahardcomposite pellet having a core of flux material surrounded by a coatingof flux-free iron ore. This application is a division of applicationSerial Number 143,096, filed October 5, 1961. r

The most common way of producing metallic iron from iron ore involvescharging-iron ore into a blast furnace along with a fiuxing material.The iron ore, which is an oxide of the elemental metal, is reduced tometallic iron by blowing high temperature reducing gases through theblast furnace. The fiuxing material is used to promote fusion ofimpurities in the ore (such as alumina, silica, etc.) and to cause thefusion to take place at a lower temperature than is required to meltsuch materials by themselves. The fluxing material is usually limestoneand/or dolomite. In recent years as reserves of high grade iron ore havebecome depleted increasing use has been made of relatively low gradeores. Such low grade ores are utilized by first grinding the ore, thenconcentrating the ore followed by forming the concentrated ore intopellets. In this invention and the prior art, about /2 of one percent ofbentonite is added to ground material to provide better water bounpellets. The pellets are dried and burned to give them sufficientstrength to withstand handling, shipping and charging into a blastfurnace. A considerable load is applied when such pellets are placed ina blast furnace and come under the loading pressure of a'tall column ofthe material. The pellets must have considerable strength when in theblast furnace to avoid the lower layers of pellets bein crushed by theweight of pellets above them, which if permitted to occur would tend tomake the charge impervious to the passage of reducing gases that must beblow through the charge to reduce it. Ores occurring in nature that arerelatively free of fluxing material have been successfully agglomeratedand burned to provide pellets of considerable strength that aresatisfactory for charging a blast furnace. One such process that hasbeen especially successful in producing pellets of outstanding strengthand great density while also being sufliciently porous for efilcientreduction in the blast furnace, is disclosed in U.S. Patent 2,925,336,William F. Stowasser, In, Feb. 16, 1960. However, it is considereddesirable to have some or even all of the fiuxing material needed duringreduction of iron ore, in the pellet when it is charged to the blastfurnace. Having all of the fiuxing material needed during reduction ofiron ore in the pellet itself has the obvious and economicallysignificant advantage of requiring fete components to be fed to a blastfurnace. Furthermore, having a fluxing material right in the pelletprovides a uniform distribution of flux and iron ore that can never beachieved by charging alternate layers of iron ore and fluxing materialinto a blast furnace.

According to the present invention, water bound composite pellets ofapproximately one-half inch diameter are made each having anapproximately A inch core surrounded by a coming approximately /3 inchthick. The inch core comprises approximately 13 percent by volume of thecomposite pellet. The inner core consists of iluxing material and theouter coating is composed of only flux-free iron ore. The material madeinto 3,,l88dh5 Fatented June 8, 1%65 the core in this example isfluxground to a size of ercent minus 200 mesh and rolled into cores 4 inchin diameter. The cores are then rolled in fiux-free'ore to apply theapproximately /a inch flux-free iron ore coating packed to the degreethat the coating is permeable to water vapor and C0 The dimensionsdescribe provide a composite pellet in which there isby weightapproximately 8 percent flux to 92 percent iron I ore, which for theparticular ore involved was the desired relationship for :blast furnacefeed in order to flux not only the siliceous component of the iron orebut also the siliceous component (the ash) inthe coke required forreduction. The described pellets are then dried at a rate that is fastenough to vaporize the mois ture within the inner core and permit thevapor to escape through the outer coating but the rate is also slowenough to heat escaping vapor only to a pressure below that which willfracture the coating. Then, after the vapors have escaped, the pelletsare further heated to a temperature above the drying temperature butbelow the incipient melting temperature of the ore, to ef-' fecthardening of the pellets outer coating. It has been found desirable tomaintain the maximum temperature of the pellets during this finalhardening to approximately 22084400 degrees Fahrenheit since theincipient melting temperature of most iron ore is about 2580 degreesFahrenheit. In this pellet there is no problem of slag (calcium and/ormagnesium .ferrites) penetrating the outer shell of iron ore at hightemperature, i.e., 2400 degrees Fahrenheit. This is' so because thecentral core of tiuxstone begins to decompose at about 160.6 to 1800degrees Fahrenheit with attendant loss of carbon dioxide (CO to shrinkin volume thus breaking contact with the iron ore and thus interruptingthe formation of slag. The interruption of slag formation despite thehightemperature, of course, represents one of the primary objects of theinvention. i 7

Other important objects of the present invention include providing newand'improved green Water bound pellets, heat hardened pellets, pelletshaving iron in easily reducible forms, and methods all leading towardimproving overall techniques for processing and convert.- ing mineralores into more useful forms." W

Other and more specific objects and how they are' achieved will appearfrom the following description when read together with the accompanyingdrawings,'in which: FIG. 1 is a photograph showing the interior of aheat hardened pellet according to present inventions, the pellet beingready for use as blast'furnace feed, and the photograph having beentaken of an image magnified six times; 1

FIG. 2 is a photograph showing the coating of a heat hardened pelletaccording to a preferred embodiment of the present invention, andbridged hematite grains'that impart strength and abrasion resistance;and p FIG. 3 is an embodiment of apparatus for carrying out the presentinvention. a

With reference to FIGS. 1 and 3, a process will be described that can beperformed on the apparatus of FIG. 3 to produce a pellet as shown inFIG. 1. FIG. 1 is labeled to point out the core and outer coating. Thisfigure will be discussed at. greater length after first describing aprocess and apparatus shown in FIG. 3. The apparatus of FIG. 3 (alongwith other apparatus) is described and claimed in a copendingapplication Serial No. l43,097, a

This loss of gas causes the inner core These small cores grow larger asthey roll through the drum. The rate of feed, the slope of the ballingdrum,

the rate of rotation of the drum and the quantity of water delivered inthe form of a spray within the drum are the design parameters that mustbe coordinated to provide the desired core formation within the drum 3.The cores discharged from drum 3 are screened to provide the desiredsize, as for example, a diameter of A inch. This sizing may beaccomplished by depositing the cores discharged from drum 3 on ascreening device Sthat delivers properly sized cores to a conveyor 6 anddischarges undersize cores to a conveyor 7. Undersize pellets depositedon the conveyor 7 may be recycled through the system so that thismaterial'is ultimately used in the system.

A coating of flux-free ore is packed about the cores to provide acomposite pellet by delivering the properly sized cores on conveyer 6 toa reroll drum iii. A hopper 11 provides a supply of flux-free iron orewhich is funneled in controlled amounts to the reroll drum 10. Theflux-free ore is distributed evenly throughout the entire length of thereroll drum is) by a screw conveyer 12 mounted within a tube 13. Thetube 13 is provided with openings 14 along its entire length to depositmaterial inlthe reroll drum along its entire length. \Vithin this rerolldrum the flux-free ore is packed in the form of an outer coating aroundthe cores previously formed in the balling drum 3. As mentioned earlier,the flux-free ore is packed about the cores to the degree that the outercoating is permeable to water vapor that must be driven olf (aswill beexplained later) and also permeable to carbon dioxide gas that will begenerated during subsequent steps in the treatment that will bedescribed. A water delivery pipe 15 is provided within the reroll drumwhich may add moisture to the flux-free ore delivered from the hopper11. The moisture, if introduced into the reroll drum, must also beintroduced along the entire length of the reroll drum and must besprayed into the drum in an even finer spray than the water that isintroduced to the balling drum 3. The reason that the moistureintroduced into the reroll drum must be in the form of a very fine sprayis that although it may be desired to increase the moisture content ofthe material in the reroll drum 10, it is not desired that additionalcores be formed within that drum. It is only desired that outer coatingsbe applied to the cores previously formed in drum 3. The designparameters for achieving a coating of, for example, a thickness ofapproximately inch and with the desired permeability may include therate of feed to the reroll drum, the slope of the drum, the speed ofrotation, and the moisture content of the coating material in the rerolldrum. These parameters can be coordinated to provide a coating on thecores with the permeability described.

The composite pellets formed by applying an outer coating in reroll drum10 to the cores that were formed in balling drum 3, may be dischargedfrom the reroll drum 10 to a screening device 17 that sizes thecomposite pellets to the desired size as per the example previouslyreferred to, /2 inch in diameter. The proper composite pellets aredischarged from the screening device 17 to a conveyer 18 that carriesthe pellets to a treating furnace 20. The treating furnace 20 includesstructures that define four separate treating zones. Hood structure 22and internal bafiling 23 define three zones 24, 25 and 26 while a rotarykiln 27 defines the fourth zone numbered 28.

Zone 24 is a preliminary drying zone, zone 25 a final drying zone, zone26 a preburning zone, and the fourth and final zone 28 is a finalburning zone. The structure shown that will be described as definingsuch zones is particularly capable to handle green pellets (bound bywater and a small amount of bentonite) fed to this furnace in a very Wetcondition. In many, if not most installations, the predrying zone 24 maynot be required. To describe apparatus capable of operation under themost adverse conditions, the furnace 20 will be described as includingthe predrying zone 24.

Composite pellets from the conveyer 18 are carried through the threezones Within the hood 22 by a gas permeable conveyer 31. Pellets aredeposited on the conveyer 31 to move as a body through zones 24, 25 and26 r with individual pellets being, relatively speaking, at rest withinthis moving body. From the conveyer 31, the pellets are discharged downan incline 32 and are fed into the rotary kiln 27. Pellets aredischarged from the kiln 27 into a cooling device such as shown at 33.There are many types of cooling devices that can be used depending onthe size of the installation. The cooling device 33 is of relativelysimple construction and may be adequate for relatively small operations.Other well-known types of coolers will be used for large installations.The cooler shown comprises a rotating, vertical shaft 34 that contains adownwardly moving column of pellets discharged from kiln 27. A blower 35blows cooling air upwardly through the descending column of pellets tocool the pellets and preheat the ascending air which is admitted to thefiring hood 36 of the kiln 27. Pellets discharged from the lower end ofthe cooler 33 may be transported away from the installation as desired.

A burner 49, projecting through burner hood 36, provides for a flamewithin the kiln 27. Hot gases proceed through the kiln 27 and the zone28 defined therein and pass into zone 26 within the hood structure 22..From the zone 26 the hot gases are drawn downwardly through the pelletsand the conveyer 31 into a suction box 41 below the grate. From thesuction box 41 the hot gases pass through a conduit 42 to zone 25. Herethe hot gases make a second pass downwardly through the pellets on theconveyer 31 and are collected in a second suction box 43. The hot gasespass from the second suction box 43 through a conduit 44 that leadsthese gases to a windbox 43 beneath zone 24. Here the hot gases passupwardly through the pellets on the traveling grate 31 into zone 24 andthey are exhausted through a conduit 46. The how of gases may bepromoted by such as an exhaust fan (not shown) arranged to draw gasesout through conduit 46.

In this embodiment shown in FIG. 3 as previously mentioned, it isassumed that the pellets are quite wet and require two-stage drying.Thus, it an apparatus providing such two-stage drying, the wet pelletsdeposited upon the traveling grate 31 will move into andthrough zone 24.As the pellets pass through this preliminary drying zone warm gases willpass upwardly through the pellets on the grate and out conduit 46. Whena preliminary drying zone is provided, as here shown, because ofexceptionally wet pellets, it 'is preferred that the gases passingthrough the pellets in the first zone be directed in an upwardlydirection rather than in a downflow direction as will be subsequentlydescribed for final drying and preburning. The reason for providingupflow preliminary drying in the first zone is that it is necessary tocarry the maximum amount of water away from the pellets in the lowerlevels of the pellets on the grate and to do so as quickly as possible.If a downflow of gases were used in a first zone for preliminary dryingof very wet pellets an even greater concentration of water would resultat the bottom of the body of pellets and in this very wet environmentthe green, relatively weak pellets could be easily squashed. This wouldnot only destroy the shape and composition that so much trouble has beengone to to provide, but also the permeability of the body of pellets onthe grate would be destroyed and further gas flow could not find its waythrough the mass of pellets on the grate. For such reasons, therefore,an upward flow of hassles gases through a first drying zone, when verywet pellets are handled, is preferred.

In a final drying zone 25 (which in many installations may be the firstzone over conveyor 31) pellets are carried through the zone and dryinggases are directed downwardly through the pellets on the travelinggrate. Substantial drying (at temperatures of 500900 degrees Fahrenheit)of the pellets should be achieved before they are permitted to leavethis zone. Thus by proper control of the speed of the conveyor 31, thepellets must be dried thoroughly but at a slow enough rate that willinsure water vapor having an opportunity to get out of the pelletwithout fracturing the outer coating. Dry pellets are ready to becarried through the preburning zone 26.

Within the preburning zone the temperature of the pellets will be raisedsufiiciently so that any magnetite that is present in the iron ore willthermally convert to hematite. Such a conversion takes place at about orapproximately between 1600 and 1800 degrees Fahrenheit. Thistransformation can be symbolically expressed by the formula 4Fe O +O 6FeO Pellets entering the preburning zone 26, although dry, will havelittle physical strength. Sutficient physical strength must be impartedto these pellets within the preburning Zone so that they can bedischarged to the final burning zone where they are tumbled. By the timethe pellets in zone 26 reach 1600 degrees Fahrenheit any magnetite thatis present will be at least superficially oxidized to hematite. Theheating of particles of hematite in this temperature range causesindividual grains of hematite in the outer coating to begin to bridgetogether by grain growth and intergranular bridging in the solid statewithout any reaction with any of the available silica or flux (flux isavailable only in the core). After individual grains have so begun tobridge together in the outer coating but before a complete network ofsuch bridged grains is completed the body of pellets in zone 26 isdisrupted and discharged into the zone 28 within the'kiln 27 whereinthey are tumbled during their final heat treating. FIG. 2 shows what thebridged hematite grains look like so that the aforementioned conditionscan be recognized. While the apparatus and process should be socontrolled to insure the beginning of such bridging, it is alsoimportant that the body of pellets be disrupted before the completenetwork of bridged grains is achieved. A densified outer coating aroundthe core that is highly resistant to degradation only if the finalbuilding of this network is caused to occur while the pellets arerolling and tumbling within the kiln 27. The temperatures required toconvert magnetite to hematite and initiate the bridging of grains togive the pellets suflicient strength to withstand rolling and tumblingis not quite high enough to cause the liquid phaseof the slaggingconstituents to occur. If the rolling and tumbling of the pellets isbegun before the temperature that causes the liquid phase of theslagging constituents is reached, then when the liquid finally does formit will be acted upon equally in all directions by the force of gravity.Very little slag will form and it will not be a problem. This is sobecause the central core of fiuxstone begins to decompose at about 1600to 1800 degrees Fahrenheit with attendant loss of carbon dioxide (COThis loss of gas causes the inner core to shrink in volume thus breakingcontact with the iron ore and thus interrupting the formation of slag.

At the same time that the force of gravity is thus being neutralized bythe rolling action of the pellet, the rolling and tumbling action of thepellet further results in the densification of the outer coating of thepellet because the pellet is exposed to a pounding action while thenetwork of bridged grains is still forming. Once the network has beencompleted it is then too late to achieve this densification. Theimportance of the proper timing of the transfer of pellets from thegrate to the kiln, to obtain the very best pellets, can be appreciated.

Thus it can be seen that the inventor has made significant and importantadvances in this art. Since the concepts and techniques that theinventor herein teaches relate to novel articles of manufacture both inthe form of green water and bentonite bound pellets and heat treated andhardened pellets and process concepts, many variations may occur tothose skilled in the art that will be within the spirit of the inventioncontributed by this invention. It is not therefore intended that theinvention described should be limited to the particular examplesdiscussed but rather that the invention should be considered as definedonly in the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. For making a strong pellet from iron ore and flux material, theprocess comprising: agglomerating a mixture of moisture and finelydivided carbonate flux material free of added iron ore to provide amoist core; forming a composite pellet by loosely packing around saidmoist core an outer coating of moist iron ore free of added fluxmaterial and compressing said coating being packed about said core tothe degree that said coating is permeable to vapor of said moisture anda dioxide gas of the carbon in said carbonate; shrinking said core awayfrom said outer coating by heating said pellet to a carbonate calciningtemperature which will produce and drive ofi from said core a vapor ofsaid moisture and a dioxide gas of the carbon in said carbonate, withsaid heating for shrinking said core being applied at a rate less than arate observed to heat escaping vapor and gases to a pressure that willfracture said coating; and then after said vapor and gases have beendriven off, then further heating said pellet above the dryingtemperature to effect hardening of said pellet.

For making a strong pellet from iron ore and flux material, the processcomprising: agglomerating a mixture of moisture, a binder and finelydivided carbonate flux material free of added iron ore to provide amoist core; forming a composite pellet by loosely packing around saidmoist core an outer coating of moist iron ore free of added fluxmaterial and compressing said coating being packed about said core tothe degree that said coating is permeable to vapor of said moisture anda dioxide gas of the carbon in said carbonate; shrinking said core awayfrom said outer coating by heating said pellet to a carbonate calciningtemperature which will produce and drive off from said core a vapor ofsaid moisture and a dioxide gas of the carbon in said carbonate, withsaid heating for shrinking said core being applied at a'rate less than arate observed to heat escaping vapor and gases to a pressure that willfracture said coating; and then after said vapor and gases have beendriven off, further heating said pellet above the drying temperature toeffect in said coating a network of bridged hematite grains.

3. For making strong pellets from iron ore and flux material, theprocess comprising: agglomerating a mixture of moisture, a binder andfinely divided carbonate flux material free of added iron ore to providea moist .heat said pellets to vaporize said moisture and drive off saidmoisture at less than a rate that is observed to develop a pressure thatwill fracture said coating; shrinking said core away from said outercoating by conveying said body through said second zone to further heatsaid pellets to a carbonate calcining temperature which will form anddrive ofl a dioxide gas of the carbon in the carbonate and to initiatebridging of adjacent grains of hematite; and then before a continuousnetwork of bridged hematite grains is achieved throughout the coating ofeach pellet, disrupting said body of pellets and tumbling said pelletsthrough said third zone While heating said tumbling pellets to a rangeabove the temperatures of said first and second zones but below theincipient melting temperature of the ore in said coating, until bridgingof said hematite grains yields a network in the coating of each pellet.

4. For making strong pellets from iron ore and flux material, theprocess comprising: agglomerating a mixture of moisture, a binder andfinely divided carbonate flux material free of added iron ore to providea moist core; forming a composite pellet by loosely packing around saidmoist core an outer coating of moist iron ore free of added fluxmaterial and compressing said coating being packed about said core tothe degree that said coating is permeable to vapor of said moisture anda dioxide gas of the. carbon in said carbonate; establishing at leastdrying, preburnirig and final burning zones; forming said, compositepellets into a movable gas permeable body with said pellets at restrelative to each other within said body; conveying said body of pelletsthrough said drying zone to heat said pellets to approximately 500900degrees Fahrenheit to vaporize and drive off said moisture at a rateless than a rate that is observed to heat escaping vapor to a pressurethat will fracture said coating; shrinking said core away from saidouter coating by conveying said body through said preburning zone tofurther heat said pellets to approximately 1600-1800 degrees Fahrenheitto form and drive off a dioxide gas of the carbon in the carbonate andto initiate bridging of adjacent grains of hematite; and then before acontinuous network of bridged hematite grains is achieved through thecoating of each pellet, disrupting said body of pellets and tumblingpellets in said final burning zone to a range (approximately 2200-2400degrees Fahrenheit) above the temperatures of said drying and preburningzones but below the incipient melting temperature (approximately 2500degrees Fahrenheit) of the ore in said coating, until bridging of saidhematite grains yields a network in the coating of each pellet.

References Cited by the Examiner UNITED STATES PATENTS 2,127,632 8/38Najarian 75-3 2,925,336 2/60 Stowasser 75-3 BENJAMIN HENKIN, PrimaryExaminer.

1. FOR MAKING A STRONG PELLET FROM IRON ORE AND FLUX MATERIAL, THEPROCESS COMPRISING: AGGLOMERATING A MIXTURE OF MOISTURE AND FINELYDIVIDED CARBONATE FLUX MATERIAL FREE OF ADDED IRON ORE TO PROVIDE AMOIST CORE; FORMING A COMPOSITE PELLET BY LOOSELY PACKING AROUND SAIDMOIST CORE AN OUTER COATING OF MOIST IRON ORE FREE OF ADDED FLUXMATERIAL AND COMPRESSING SAID COATING BEING PACKED ABOUT SAID CORE TOTHE DEGREE THAT SAID COATING IS PERMEABLE TO VAPOR OF SAID MOISTURE ANDA DIOXIDE GAS OF THE CARBON IN SAID CARBONATE; SHRINKING SAID CORE AWAYFROM SAID OUTER COATING BY HEATING SAID PELLET TO A CARBONATE CALCININGTEMPERTURE WHICH WILL PRODUCE AND DRIVE OFF FROM SAID CORE A VAPOR OFSAID MOISTURE AND A DIOXIDE GAS OF THE CARBON IN SAID CARBONATE, WITHSAID HEATING FOR SHRINKING SAID CORE BEING APPLIED AT A RATE LESS THAN ARATE OBSERVED TO HEAT ESCAPING VAPOR AND GASES TO A PRESSURE THAT WILLFRACTURE SAID COATING; AND THEN AFTER SAID VAPOR AND GASES HAVE BEENDRIVEN OFF, THEN FURTHER HEATING SAID PELLET ABOVE THE DRYINGTEMPERATURE TO EFFECT HARDENING OF SAID PELLET.